In a membrane separation process for activated sludge, which is one of processes of wastewater, a membrane filter element is immersed in an activated-sludge tank and activated sludge is separated by filtration. In this process, the filtration can be carried out at a significantly high concentration of mixed liquor suspended solid (MLSS) of 5000 to 20000 mg/L. Accordingly, the process has advantages in that the volume of the activated-sludge tank can be reduced or the time for reaction conducted in the activated-sludge tank can be shortened. In addition, since the filtration is carried out through a membrane, a final settlement tank for removing suspended solids (SS) from treated water is unnecessary. Furthermore, since the filtration can be carried out regardless of sedimentation properties of activated sludge, no special countermeasure is necessary even if the sedimentation properties of activated sludge are poor. Thus, the membrane separation process has many advantages compared to sedimentation processes and, therefore, has been rapidly developed recently.